Before two modems can begin to communicate over a communications channel, the modems must first confirm that an adequate connection exists and establish mutually acceptable ground rules for communication. To do so, when a connection is first established between the modems, the modems will engage in a “training sequence” designed to both test the characteristics of the telephone circuit and select an agreed communication protocol. The training sequence is often referred to as “handshaking,” by analogy to the human “handshake” that commonly occurs before two people begin to communicate with each other.
The modem handshaking sequence is particularly important, because numerous different modulation protocols or standards exist (defining aspects such as the “handshake method” (XON/XOFF), the baud rate, the parity setting, the number of data bits, and the number of stop bits), and the modems need to establish up front which protocol will be used. Further, the line conditions may preclude or require the use of one or more protocols or protocol-settings, and the modem handshaking sequence can be used to measure those line characteristics and facilitate a determination of what limitations or requirements exist.
1. Exemplary Modem Training Sequence
To initiate communication between two modems on a common telephone circuit, the originating modem first places a call to the answering modem, usually by dialing the phone number of the answering modem over the public switched telephone network (PSTN). The answering modem detects a ring on the telephone line and responsively goes off-hook to answer the call. The answering modem then begins the modem handshaking sequence. By convention, the answering modem begins this handshaking sequence by sending an answer tone of a predefined frequency to the originating modem, which may specify in simple terms (typically by its presence only) a proposed modulation protocol.
An answer tone was originally included in the handshaking protocol so as to disable echo suppressors and echo cancellers in the telephone network. However, as different modulation protocols have evolved, characteristics of the answer tone have been used to identify the presence of specific protocols. By way of example, phase reversals in the answer tone may serve to identify v.32 and v.34 echo-cancelled modems.
According to traditional analog modem communication protocols, such as V.32 and V.34, when the originating modem recognizes the answer tone, it responsively turns on its own transmitter and sends an originating mark carrier of a predefined frequency to the answering modem to confirm its availability to communicate according to the specified protocol. Alternatively, the originating modem may negotiate for the use of some other mutually acceptable protocol. Once the two modems agree on a modulation standard and once they have completed training, the modulated communication of useful information may begin.
According to more recent digital transmission protocols, the “training sequence” may take on other functions. For instance, according to the V.90 standard, after the answering modem sends the answer tone to the originating modem, the modems engage in several stages of a startup sequence, including (i) V.8 or V.8bis negotiation to identify V.90 capabilities and modes of operation (e.g., data mode, text mode, and modulation mode), (ii) line-probing to determine if the telephone circuit can support V.90 transmissions, (iii) training of equalizers and echo cancellers and testing the line for distortion, and (iv) exchange of constellation coding information to facilitate communication. The functions and operation of this training process are well known to those of ordinary skill in the art and need not be described in great detail here.
In general, during the line-probing stage, the originating modem and the answering modem transmit to each other probing signals that are made up of complex signals. When each modem receives the probing signal, it may analyze the signal in order to determine characteristics of the subscriber line. Based at least in part on these characteristics, the two modems can then determine whether a V.90 compatible connection is possible or whether a different protocol, such as V.34 or V.32 should be employed instead.
Further, based on an analysis of the respective probing signals that they receive, either or both modems may adjust various operating parameters, in order to optimize communication. As a result, for instance, the modems may adjust the carrier frequency and symbol rates that the modems use for signal transmission. As another example, the modems may engage in an adaptive pre-emphasis process in order to adjust (e.g., spectrally shape) various parts of the signal being transmitted so as to reduce effects of signal-dependent distortion. As still another example, the modems may engage in an adaptive power-control process so as to increase transmission power and increase the signal-to-noise ratio (preferably without introducing substantial echo distortions).
After line probing, the modems train equalizers and echo-cancellers and make measurements (such as signal levels and quantization distortions) that enable one or both modems to determine what impact if any the communication link between the modems has on digital transmissions. This latter procedure is known in the art as a “Digital Impairment Learning” or DIL, and it is commonly used in V.90 compatible communication systems, in which one communication end is terminated with a digital modem and a second communication end is terminated with an analog modem. In a V.90 compatible communication system, a digital modem typically transmits a DIL signal to an analog modem, which receives and processes the DIL signal.
To initiate the DIL process, the analog modem transmits to the digital modem a signal describing a sequence of codewords that the analog modem expects to receive. The description of the sequence of codewords is known as a DIL-descriptor, and the signal is known as a “Ja” signal. When the digital modem receives the “Ja” signal and builds the DIL sequence described by the DIL-descriptor in the “Ja” signal, the digital modem repeatedly transmits to the analog modem the DIL sequence that the analog modem expects to receive.
A DIL-sequence may be vendor-specific; no particular sequence is required by the standards. Thus, for example, of the 128 possible unique 8-bit PCM codewords, the DIL sequence could consist of 117 unique PCM codewords. A predefined signal, such as an “S/Sbar” signal, can then signify the end of the “Ja” signal (and the beginning of the next phase of the training sequence). In general, the “S” signal is a simple predefined signal consisting of two alternating signal points that are 90 degrees apart, and the “Sbar” signal is a signal generated by phase shifting the “S” signal by 180 degrees. The “S” signal and the “Sbar” signal are defined in the V.34 International Telecommunication Union-Telecommunication (“ITU-T”) standard, which is incorporated herein by reference.
When the digital modem receives the “Ja” signal, the digital modem transmits the DIL sequence described by the DIL-descriptor in the “Ja” signal. Then, when the analog modem receives the codewords of the DIL sequence, the analog modem can collect and store the corresponding amplitudes in a table of 128 rows by 6 columns. Each row of the table may correspond to a particular PCM codeword of the DIL sequence, and each column of the table may correspond to a particular phase in which the symbol was received by the analog modem. Based on some or all entries in this table, the analog modem may then perform DIL processing in a number of stages. For instance, the analog modem might be arranged to perform the DIL processing in two stages.
First, the analog modem seeks to determine what encoding law (e.g., μ-law or A-law) the digital end used to generate the PCM codewords that it transmitted and the extent to which the communications link digitally attenuated the signal. To do this, the analog modem formulates a hypotheses as to what scaling factors the line applied to the signal, and the analog modem then tests the hypotheses by decoding, the values in the table, first with a scaled μ-law compander and then with a scaled A-law compander. The analog modem then compares each decoded value to the value that it expected to have been transmitted by the digital modem and thereby derives a decoding error for each value. A combination of the decoding errors for some or all of the received codewords provides a measure of veracity of the original hypotheses. By performing this process over a number of different hypotheses, each representing a respective scaling value, the analog modem can identify the encoding law and a scaling value that produces the least error and, thus, the analog modem can select a winning hypothesis.
Second, after receiving all of PCM codewords of the DIL sequence, the analog modem maps the transmitted codewords (i.e. expected codewords) with the received codewords. In particular, for instance, the analog modem can generate a number of bitmaps representing the transmitted and respective received codewords. The analog modem may then select a subset of transmitted codewords that maximizes the Euclidean distance between received values and that matches the noise ratio of the channel and the desired error rate. The analog modem may perform this process for several phases and then, in the final stage of the training sequence, may transmit the subsets to the digital modem.
In typical practice, the entire modem training sequence takes on the order of twenty to thirty seconds to complete. During this time, the modems do not communicate user data. Therefore, it is desirable to reduce the length of the modem training sequence.
2. Proposed Technique to Shorten the Training Sequence
In many environments, such as in accessing an Internet Service Provider (ISP), a network device such as an analog modem typically establishes a connection with an answering modem over the same subscriber line every time that it connects with the answering modem. Consequently, information that the originating modem and the answering modem obtain about the characteristics of that line during an initial training sequence may be equally applicable in later communication sessions. In a given communication session, if one of the modems can somehow determine that the transmission line characteristics are substantially the same as they were in a previous session, the modems should theoretically be able to cut short the training sequence, omitting some or all of the lengthy line-characteristic analysis described above.
According to a method proposed by Conexant Systems Inc., one way to determine whether a shortened training sequence can be employed is to effectively superimpose a line probing signal on the answer tone that the digital modem initially sends to the analog modem. Conexant refers to this combination signal as an “ANSpcm signal.”
More particularly, according to Conexant, the digital modem retrieves from a stored table and transmits to the analog modem a sequence of 320 predefined PCM codewords (consisting of 70 unique codewords) that represent the answer tone signal. Concurrent with its receipt of the answer tone, the analog modem may then compare the received codewords with a stored table of the predefined 320 PCM codewords and, as in the DIL process described above, determine the characteristics of the line.
In this way, the analog modem can effectively kill two birds with one stone: (i) it can receive the answer tone and, simultaneously, (ii) receive a probing signal that allows it to learn the line characteristics so that it can determine whether the connection is the same as before. If the line characteristics are the same as before, the analog modem can then use the same receiver settings, without having to engage in the full training sequence. The modems can then more quickly enter a useful communication session.